The coding gain (a C/N difference before and after error correction that achieves a necessary bit error rate (BER)) of the forward error correction (FEC) method applied to wireless communication systems is desirably high. Therefore, wireless communication systems are beginning to adopt highly coding gain codes (e.g., a turbo (convolutional) code, turbo product code (TPC), and LDPC code) that have been extensively studied in recent years. Since these highly coding gain codes are adopted, systems can be used at a C/N lower than that when using the RS (Reed-Solomon) code as a representative code adopted in the conventional systems. Another feature of these highly coding gain codes is that the code length is longer than those of codes such as the RS code in order to increase the coding gain.
On the other hand, the reception level of a wireless communication system fluctuates in accordance with the status of a transmission channel. In many wireless communication systems, therefore, a protection channel is formed for an important channel, and the regular channel is switched to the protection channel by using a channel switching apparatus if the condition of the regular channel deteriorates. For example, Japanese Patent Publication No. 8-4257 (reference 1) describes a channel switching apparatus capable of well following the quality deterioration speed of a propagation channel by monitoring the channel quality before error correction even in a system having an error correcting function.
An error correction code used in the conventional wireless communication systems is generally a linear block code (e.g., the RS code or BCH code) to be decoded on the basis of the bounded distance decoding method using a hard-decision signal alone. When using these linear block codes, a decoder executes decoding only once for each code block. In addition, during the course of a decoding operation, the decoder outputs a syndrome signal indicating the presence/absence of errors in a transmitted block, and an error bit correction signal to be used when correcting error bits. The error rate information of the transmission channel can be obtained on the basis of the occurrence probability (i.e., the number of times of occurrence within a predetermined time) of these signals (the syndrome signal and error bit correction signal). The regular channel and protection channel are switched on the basis of this error rate information.
Also, since the coding gain of the linear block code as described above is not so high, a state in which the BER after error correction is about 1×10−6 can be detected on the basis of the syndrome signal or error bit correction signal.
Furthermore, in wireless communication systems, channel switching is performed when the channel quality matches a predetermined condition. Generally, channel switching is executed when the BER has exceeded a predetermined value (e.g., 1×10−6). In this case, a system in operation cannot directly measure the BER. In a system having an error correcting function, therefore, the syndrome signal or error bit correction signal as the error rate information output by an error correction decoder during the operating process is observed for a predetermined time, and the BER is estimated by the number of observed signals.
The syndrome signal takes a value 0 if there are no errors bit in a code block, and a value 1 if there is even one bit error.
Also, the conventional method such as the RS code detects the positions and bit patterns of errors from a decoding operation result corresponding to a hard-decision signal, and executes error correction by inverting a decoder input signal. In this case, the signal for bit inversion is an error bit correction signal. That is, when error correction is correctly executed, the error bit correction signal is a pulse signal corresponding to the number of bits of the errors.
Assume that when using the syndrome signal as the error rate information, the code length is n bits, and the BER before correction equivalent to the BER of the switching condition after correction is p. In this case, the occurrence probability of the syndrome signal can be obtained by calculating the probability of the occurrence of an error having one bit or more in the n bits from p. By detecting a state equal to this syndrome signal occurrence probability, a switch signal can be output when the BER of the switching condition is detected. Note that even when using the error bit correction signal as the information source (error rate information), a switch signal can be output by performing control in accordance with the same concept as above.
A channel switching signal generating circuit in the wireless communication system described in, e.g., reference 1 will be explained below with reference to the accompanying drawings. FIG. 11 is a block diagram showing an example of the arrangement of the channel switching signal generating circuit used in the wireless communication system. In this example shown in FIG. 11, the channel switching signal generating circuit includes a decoder 904, counter 905, and comparator 903.
Referring to FIG. 11, the counter 905 receives a syndrome signal or error bit correction signal (“an error correction control bit” in reference 1) output from the decoder 904. Also, the counter 905 counts the input signals (syndrome signals or error bit correction signals) within a set observation period. The counter 905 then outputs the input signal count result to the comparator 903. The comparator 903 compares a threshold value with the output (count result) from the counter 905, and outputs a switch signal.